Strengthening the Reporting of Observational Studies in Epidemiology (STROBE): Explanation and elaboration by Vandenbroucke, J, von Elm, E, Altman, D, Gøtzsche, P, Mulrow, C, Pocock, S, Poole, C, Schlesselman, J, Egger, M, Blettner, M, Boffetta, P, Brenner, H, Chêne, G, Cooper, C, Davey-Smith, G, Gagnon, F, Greenland, P, Greenland, S, Infante-Rivard, C, Ioannidis, J, James, A, Jones, G, Ledergerber, B, Little, J, May, M